A monomer is a molecule or family of similar molecules that are strung together in sequence to form a macromolecule called a polymer. The two common types of polymers in bioinformatics are nucleic acids (for which the monomers are nucleotides) and polypeptides (whose monomers are amino acids), which make up proteins.

In each of these cases, the polymer is formed by a series of dehydration reactions, in which a hydrogen atom (H) is taken from one monomer and a hydroxyl group (OH) is taken from the other to form a single molecule of water, which is split off from the growing polymer.